[Bug 1026990] [NEW] Add new option to dhclient to send DHCP requests on a cerrtain interface

[Mike Harris]

Public bug reported:

We have an interesting situation with dhclient, multiple interfaces, and
DHCP servers not on the same subnet as a network.

The setup:
- on a machine with two connected interfaces, eth0 and wlan0.
- the DHCP server for wlan0 is not on the same network segment as its clients.

When it's time to renew the lease for wlan0, dhclient will try to renew
via unicast to the DHCP server. Since it's on a different segment, the
kernels routing tables are used to decide what interface the packet is
sent out over. This could result in a DHCP request for wlan0 getting
sent out over eth0, which is undesirable.

The DHCP standard doesn't address this problem, so it's not really a
bug, but it would be nice to resolve. This issue came up when we noticed
errors on some switches.

To resolve this, we're thinking this might work:
a) Add a new flag to dhclient so that dhclient only sends IP packages via the interface the running instance is responsible for (ignoring the routing table).
b) Add an option to network-manager which passes this flag to dhclient

** Affects: isc-dhcp (Ubuntu)
     Importance: Undecided
         Status: Confirmed

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https://bugs.launchpad.net/bugs/1026990

Title:
  Add new option to dhclient to send DHCP requests  on a cerrtain
  interface

Status in “isc-dhcp” package in Ubuntu:
  Confirmed

Bug description:
  We have an interesting situation with dhclient, multiple interfaces,
  and DHCP servers not on the same subnet as a network.

  The setup:
  - on a machine with two connected interfaces, eth0 and wlan0.
  - the DHCP server for wlan0 is not on the same network segment as its clients.

  When it's time to renew the lease for wlan0, dhclient will try to
  renew via unicast to the DHCP server. Since it's on a different
  segment, the kernels routing tables are used to decide what interface
  the packet is sent out over. This could result in a DHCP request for
  wlan0 getting sent out over eth0, which is undesirable.

  The DHCP standard doesn't address this problem, so it's not really a
  bug, but it would be nice to resolve. This issue came up when we
  noticed errors on some switches.

  To resolve this, we're thinking this might work:
  a) Add a new flag to dhclient so that dhclient only sends IP packages via the interface the running instance is responsible for (ignoring the routing table).
  b) Add an option to network-manager which passes this flag to dhclient

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